Patience and persistence can pay off sometimes, but the likelihood of getting a set of Bronson for £20 -as a friend of mine once did, the bastard- is pretty remote!  Still, a lot of standard texts do turn up at affordable prices if you're not too worried about the state of the bindings.

So far as books in print in Britain and Ireland are concerned, there is a list of some of the more authoritative collections which are currently in print, here:  Dave Herron's Chapbook.  It was compiled by Dave Herron (Eckersley) of the Traditional Song Forum a couple of years ago, and I've recently updated it a bit.  It is not an exhaustive list, and concentrates on the better examples, omitting the "coffee table" variety of modern anthology, but there are some useful books there.  Llanerch Publishers in particular have reprinted a number of important texts, such as Frank Kidson's Traditional Tunes, Bruce and Stokoe's Northumbrian Minstrelsy and so on, but they are going out of business in the near future, so don't delay!

I'm not up to speed on American collections in print, but there have been a few threads relating to the subject in the past where you may find further useful information.  Unfortunately, a lot of online booksellers (Amazon is a prime offender) insist on listing books as in print which, when you order them, turn out to be unavailable.

Malcolm